Track search control circuit and optical disc drive

ABSTRACT

Disclosed is a track-search control circuit for performing track-search of an optical disc stably even at the time of multi-speed reproduction of the optical disc. The track search control circuit comprises a circuit for detecting the light beam having traversed the track when the light beam moves in the radial direction of the optical disc and generating a normal direction on-track signal FVACLR at the time of being on the track and a normal direction off-track signal FVBCLR at the time of being off the track, a first measurement circuit that starts time measurement at the time of generating the normal direction on-track signal, a second measurement circuit that starts time measurement at the time of generating the normal direction off-track signal, circuits which detect the error between a moving velocity of the optical beam in the radial direction of the optical disc and a target velocity based on a measurement output by the first measurement circuit and a measurement output by the second measurement circuit, and a correction circuit for correcting the moving velocity of the optical beam in the radial direction based on the detected error signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 11-256209, filed Sep. 9, 1999,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a track search control circuit forcontrolling track search of an optical disc and an optical disc driveequipped with a track search control device with the use of the tracksearch control circuit, which are used, for example, for optical discreproducing apparatuses such as a CD (Compact Disc) ROM drive for acomputer system, a DVD (Digital Versatile Disc) drive, etc. and foroptical disc recording/reproducing apparatuses for a CD-R, a CD-RW, aDVD-RAM, etc.

Conventionally, in optical disc systems for the DVD, the CD, etc., whenthe track search of the optical disc is performed, track traversingdirection/frequency of a laser beam spot are detected using amplitudeinformation of a tracking error signal and a readout signal at a timewhen the laser beam spot traverses the track, and the velocity controlof the track search is performed.

That is, when the track search is performed, the track search controlcircuit is given the track search direction BWFW and the necessarynumber of tracks to be searched by a system controller, and generatestrack traversing information of the laser beam spot from a trackingerror signal TE and a readout ripple signal RFRP. Then,acceleration/deceleration energy necessary for the track search iscalculated from the track traversing information, the track searchdirection, and the number of tracks to be searched, and a signalindicative of the energy is added to an input of a tracking servoequalizer.

Now, along with the increase in a disc angular velocity in reproducingthe optical disc in connection with the competition for reproductionspeed of optical disc systems, the necessity of performing the tracksearch under a condition of a large angular velocity of the disc hasbeen arisen. However, in the case where the disc angular velocity islarge, the variation of eccentric acceleration of the disc is large, andhence it is likely to be difficult to control the track search velocityeffectively in the conventional track search method.

Hereafter, regarding this respect, a configuration of a conventionalvelocity error detection circuit for the track search control circuitwill be described with reference to FIG. 11.

In the velocity error detection circuit of FIG. 11, a velocity errordetection counter (FVCTR) 81 measures a semi-track interval. Thevelocity error detection counter FVCTR 81 is a 5-bit counter, whosecount is incremented by a target velocity clock FVCK. Therefore, thetarget velocity clock counter (FVCTR) 81 can count thirty-two clocks(FVCKS) and when the counter reaches the full-count, a full countdetection signal FULL-DET becomes logic “H”. Further, when the velocityerror detection counter (FVCTR) 81 counts the full count, a gate 83 forthe target velocity clock FVCK is turned off using an inverted signal ofthe full count detection signal FULL-DET by the inverter circuit 82 inorder that after the full count the full count value does not return tozero by the next target velocity clock FVCK.

On the other hand, a normal direction on-track/off-track signal FVCLR isa pulse generated at the normal direction on-tracking/off-tracking time,which clears the FVCTR 81. It should be noted that the “normaldirection” means a case where a direction in which the lens of thepick-up mechanism is moved with regard to the track is the same as atrack search direction given by a system controller.

A data latch signal FVLP is a pulse generated just before the generationof the normal direction on-track/off-track signal FVCLR. The data latchsignal FVLP latches (the count value of the FVCTR−15)×(+1) or (the countvalue of the FVCTR−15)×(−1) in the TKIC (track search velocity control)register 85. Specifically, when the track search is to be performed inthe outward direction, the BWFW signal from the system controller isgoes to a logical low level, and in this time, (the count value of theFVCTR−15)×(+1) is latched by the TKIC register 85. On the other hand,when the track search is to be performed in the inward direction, theBWFW signal from the system controller goes to a logical high level, andin this time, (the count value of the FVCTR−15)×(−1) is latched by theTKIC register 85.

data including the count value (velocity measurement data) of the FVCTR81 and the track search direction BWFW given by the system controllerinto a TKIC (track search velocity control) register 85. These normaldirection on-track/off-track signal FVCLR and data latch signal FVLP aregenerated using the tracking error signal TE and the readout ripplesignal RFRP.

Therefore, the velocity measurement result by the FVCTR 81 that countsup the target velocity clock FVCK is loaded into the TKIC register 85 atthe data latch signal FVLP generated just before the generation of thetrack pulse, and is cleared by an inverted signal of the normaldirection on-track/off-track signal FVCLR by an inverter circuit 84. Inthis case, a velocity measuring period of the FVCTR 81 is a half-trackperiod, and acceleration/deceleration data stored in the TKIC register85 is outputted in a half-track period next to the velocity measuringhalf-track period.

FIG. 12 is a characteristic diagram showing the relation between thetrack search velocity error and the number of the FVCK counts of thevelocity error detection counter (FVCTR) 81 of FIG. 11.

FIG. 13 is a characteristic diagram showing the relationship between theFVCTR value detected as the velocity error and the value stored in theTKIC register 85 in the outward direction search, i.e., BWFW=“L”.

FIG. 14 is a characteristic diagram showing the relationship between theFVCTR value detected as the velocity error and the value stored in theTKIC register 85 in the inward direction search, i.e., BWFW=“H”.

FIG. 15 is a waveform chart showing output timing for a velocity errordetection result in the velocity error detection circuit of FIG. 11 in acase where a track search in the outward direction is taken as anexample.

In FIGS. 11 and 15, the target velocity clock FVCK is a clock having afrequency sixteen times that of the actual target velocity (half-tracktraversing target frequency). Accordingly, if the FVCTR 81 countedsixteen clocks until the velocity measurement result of the FVCTR 81 islatched in the TKIC register 85 at the data latch signal FVLP, it is thecase where the actual velocity is equal to the target velocity (i.e. thevelocity error being zero), the value zero (“0”) is stored in the TKICregister 85.

Moreover, if the FVCTR 81 counted less than sixteen clocks until thevelocity measurement result of the FVCTR 81 is latched in the TKICregister 85 at the data latch signal FVLP, it is the case where theactual velocity is higher than the target velocity, and a negative valuein the outward direction search or a positive value in the inwarddirection search is stored in the TKIC register 85.

On the contrary, if the FVCTR 81 counted more than sixteen clocks untilthe velocity measurement result of the FVCTR 81 is latched in the TKICregister 85 at the data latch signal FVLP, it is the case where theactual velocity lower than the target velocity, and a positive value inthe outward direction search or a negative value in the inward directionsearch is stored in the TKIC register 85.

In this way, error data between the target velocity and the actual tracktraversing velocity is modified by the track search direction signalBWFW and stored in the TKIC register 85, which becomes an input ofaddition of the tracking servo equalizer to effect the addition of asignal indicative of acceleration/deceleration energy in a trackingdirection.

The velocity control method for the track search mentioned abovereferring to FIGS. 11 to 15 has an advantage that the track searchvelocity can be controlled accurately based on the measured velocityerror (in multiple values). However, as will be mentioned below, it isdifficult to perform the track search in the optical disc system for theDVD, the CD, etc. stably when the disc is reproduced at a multi-speed,because the timing of outputting the velocity measurement result storedin the TKIC register 85 is always in a next half-track period next tothe period (half-track) when the velocity measurement was performed andbecause there may exist the eccentricity of the axis of rotation of theoptical disc.

That is, generally in the case of a removable disc, when the disc ismanufactured, the center of the disc made in a donut-shape is notnecessarily in the center of a track formed in a spiral manner fromwhich the signal is read. Moreover, the center of a track formed in aspiral manner is not necessarily in the axis of a disc motor, whichresults from improper placement of the disc when the disc is loaded(clamped). Furthermore, in manufacturing a disc drive, it is probablethat the rotor axis of the disc motor for rotating the disc is not inthe actual axis of rotation. Moreover, the rotor axis of the disc motormay have inclination to the actual axis of rotation.

The effect of such eccentricity of the disc rotation on the track searchwill be considered. When the track search is performed in an idealcondition without the eccentricity and if an objective lens housed inthe pickup is moved at a constant velocity toward the inwarddirection/outward direction of the disc with respect to a frame fixingthe mechanism, the relative velocity between the track and the objectivelens becomes also constant. However, when the track search is performedunder a condition with the existence of the eccentricity, if theobjective lens is moved at a constant velocity with respect to theframe, the relative velocity between the track and the objective lens ismodulated by the eccentric acceleration.

Therefore, in order to keep the relative velocity between the track andthe objective lens constant or in a target velocity, it is necessary toalter the acceleration/deceleration energy that is given to a trackingcoil (a drive coil of a pickup sending motor) in response to this changeof the eccentric acceleration.

In this case, in the velocity control method as described above, thevelocity measurement is performed for every half-track period bycounting the target velocity clock FVCK and this count data is used asthe track search velocity error data (acceleration/deceleration data).However, regarding the velocity measurement, the velocity measurementresult for previous half-track period is used and theacceleration/deceleration data is outputted in a half-track period nextto the velocity measuring half-track period, and hence this method mayarise a problem.

The reason for this is that, since the variation of the eccentricacceleration is large in the multi-speed reproduction, the relativevelocity between the track and the lens for the half-track period whenthe track search velocity is measured may differ largely from that for ahalf-track period when the acceleration/deceleration data is actuallyoutputted, which results from the modulation due to the eccentricity.

That is, in an example shown in FIG. 15, at a time when the count valueof the FVCTR 81 reaches C5, the track search control circuit is going tooperate in such a way that almost maximum acceleration energy in theoutward direction is made to be outputted, however in the nexthalf-track period when the acceleration/deceleration data for outputtingthis acceleration energy is actually outputted, the track searchvelocity has already become approximately close to the target velocity.

The worst case is a case where the track search velocity is too highcompared to the target velocity at the time of the detection of thetrack velocity, and the track search velocity, namely the relativevelocity between the track and the objective lens, is controlled so asto become lower because of the erroneous measurement of the track searchvelocity resulted from the modulation due to the eccentricity in thenext half-track period when the acceleration/deceleration data forcausing the deceleration energy to output is actually outputted. In thiscase, although the track search velocity is low, the deceleration energyis to be further applied. However, since the acceleration/decelerationdata is not renewed until the light beam traverses the track, there islikely to occur a situation where the deceleration energy term is kepton to be applied. In this case, the acceleration/deceleration controlgets into an oscillation state and it may be difficult that the tracksearch is stably performed.

BRIEF SUMMARY OF THE INVENTION

The present invention has been contrived in order to solve theabove-mentioned problem, and its object is to provide a track searchcontrol circuit which can promptly reflect a detected result of thetrack search velocity at a multi-speed reproduction time on theacceleration/deceleration data, to thereby stably perform the tracksearch operation as well as an optical disc drive equipped with a tracksearch device including such a track search control circuit.

A track search control circuit according to a first aspect of thepresent invention comprises an optical pickup for emitting and moving alight beam in the radial direction of an optical disc to writeinformation signal into the optical disc or read the information signaltherefrom; a track traversing signal generation circuit for detecting,when the light beam emitted from the optical pickup moves in the radialdirection of the optical disc, the light beam having traversed a trackof the optical disc, and generating a normal direction on-track signalin an on-track period when the light beam traverses a zone of the trackin a track search direction defined by a system controller and a normaldirection off-track signal in an off-track period when the light beamtraverses a zone between the tracks; a first time measurement circuitwhich starts time measurement at a time when the normal directionon-track signal is generated by the track traversing signal generationcircuit; a second time measurement circuit which starts time measurementat a time when the normal direction off-track signal is generated by thetrack traversing signal generation circuit; a velocity error signalgeneration circuit for detecting an error between a relative movingvelocity of the light beam of the optical disc to the track and a targetvelocity based on a measurement outputted by the first time measurementcircuit and a measurement outputted by the second time measurementcircuit to generate an error signal; and a correction circuit forcorrecting the moving velocity of the light beam in the radial directionbased on the error signal generated by the velocity error signalgeneration circuit.

In a track search control circuit according to the first aspect of thepresent invention, the normal direction on-track signal and the normaldirection off-track signal may be generated approximately at the time ofzero-crossing of a tracking error signal indicating a relativepositional displacement in the radial direction of the optical discbetween the track and the light beam emitted from the optical pickup.

An optical disc drive according to a second aspect of the presentinvention comprises an optical pickup for emitting a light beam on atrack of an optical disc, on which information is recorded, andreceiving the reflected light from the track or the transmitted lighttherethrough while the optical disc is rotating, thereby extracting theinformation and converting the information to an electric signal; asignal processing circuit for generating a tracking error signal thatindicates a relative positional displacement in the radial direction ofthe optical disc between the track and the light beam emitted from theoptical pickup and a ripple signal that indicates amplitude information,from the electrical signal output of the optical pickup at a time whenthe light beam emitted from the optical pickup moves in the radialdirection of the optical disc; tracking servo mechanism for controllingthe light beam emitted from the optical pickup in response to thetracking error signal so that the light beam in the radial direction ofthe disc is positioned on the track; a track traversing signalgeneration circuit for detecting that the light beam emitted from theoptical pickup has traversed the track based on the tracking errorsignal and the ripple signal, and generating a normal direction on-tracksignal in an on-track period when the light beam traverses a zone of thetrack and a normal direction off-track signal in an off-track periodwhen the light beam traverses a zone between the tracks; a first timemeasurement circuit that starts time measurement at a time when thenormal direction on-track signal is generated by the track traversingsignal generation circuit; a second time measurement circuit that startstime measurement at a time when the normal direction off-track signal isgenerated by the track traversing signal generation circuit; a velocityerror signal generation circuit for detecting an error between a movingvelocity of the optical beam in the radial direction of the optical discand a target velocity based on a measurement output of the first timemeasurement circuit and a measurement output of the second timemeasurement circuit to generate an error signal; and a tracking velocitycorrection circuit for correcting the moving velocity of the opticalbeam in the radial direction by applying the error signal output of thevelocity error signal generation circuit to the tracking servomechanism.

In an optical disc drive according to the second aspect of the presentinvention, the tracking velocity correction circuit may start to apply asignal indicative of an acceleration energy corresponding to the errorsignal to the tracking servo mechanism in a half-track period after whenthe velocity error signal generation circuit starts the error detectionand a signal indicative of a deceleration energy corresponding to theerror signal to the tracking servo mechanism when a succeedinghalf-track comes in the target velocity period after when the velocityerror signal generation circuit starts the error detection. The tracktraversing signal generation circuit, the first time measurementcircuit, the second time measurement circuit, the velocity error signalgeneration circuit, and the tracking velocity correction circuit may beformed on the same semiconductor chip in a form of an integratedcircuit.

In an optical disc drive according to the second aspect of the presentinvention, the track traversing signal generation circuit may generatethe normal direction on-track signal and the normal direction off-tracksignal approximately at the time of zero-crossing of the tracking errorsignal.

The track traversing signal generation circuit, the first timemeasurement circuit, the second time measurement circuit, the velocityerror signal generation circuit, and the tracking velocity correctioncircuit may be formed on the same semiconductor chip in a form of anintegrated circuit.

In an optical disc drive according to the second aspect of the presentinvention, the first time measurement circuit may comprise a firstcounter which is cleared by the normal direction on-track signal, countsclock signals having a constant frequency higher than that of the normaldirection on-track signal, and goes into a hold status after generatinga first flag output indicating that the moving velocity of the lightbeam after the generation of the normal direction on-track signal islower than the target velocity; the second time measurement circuit maycomprise a second counter which is cleared by the normal directionoff-track signal, counts the clock signals, and goes into a hold statusafter having counted a specified number of clocks and subsequentlygenerating a second flag output indicating that the moving velocity ofthe light beam after the generation of the normal direction off-tracksignal is lower than the target velocity; and the velocity error signalgeneration circuit, based on the first flag output and the second flagoutput, may generate an acceleration flag when the moving velocity ofthe light beam after the generation of the normal direction on-tracksignal and the moving velocity of the light beam after the generation ofthe normal direction off-track signal are both lower than the targetvelocity, and may generate a deceleration flag when the moving velocityof the light beam after the generation of the normal direction on-tracksignal and the moving velocity of the light beam after the generation ofthe normal direction off-track signal are both higher than the targetvelocity. The tracking velocity correction circuit may apply the signalindicative of the acceleration energy or deceleration energy ofsubstantially a constant level to the tracking servo mechanism duringboth the acceleration flag and the deceleration flag are logically setup. The track traversing signal generation circuit, the first timemeasurement circuit, the second time measurement circuit, the velocityerror signal generation circuit, and the tracking velocity correctioncircuit may be formed on the same semiconductor chip in a form of anintegrated circuit.

In an optical disc drive according to the second aspect of the presentinvention, the track traversing signal generation circuit, the firsttime measurement circuit, the second time measurement circuit, thevelocity error signal generation circuit, and the tracking velocitycorrection circuit may be formed on the same semiconductor chip in aform of an integrated circuit.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a block diagram showing one example of a DVD system accordingto an embodiment of the present invention.

FIG. 2 is a view showing one example of the track search control circuitin the DVD system of FIG. 1.

FIG. 3 is a characteristic diagram showing the relation between thenumber of remaining tracks in a velocity decoder of the FIG. 2 and thecalculated results of the track search target velocity.

FIG. 4 is a characteristic diagram showing a transfer characteristicfrom the target velocity data to the actual target velocity clock in thetarget velocity clock generation circuit of FIG. 2.

FIG. 5 is a logical circuit chart showing one example of the velocityerror detection circuit of FIG. 2.

FIG. 6 is a characteristic diagram showing the relation between thenumber of FVCK counts of the velocity error detection circuit of FIG. 5and the velocity error detection signals ACRRY and BCRRY.

FIG. 7 is a characteristic diagram of a decoder of the velocity errordetection circuit of FIG. 5.

FIG. 8 is a waveform diagram showing the detection result of thevelocity error by the velocity error detection circuit and one exampleof the output timing for the velocity control output.

FIG. 9 is a characteristic diagram showing the relation between thevelocity error of the velocity error detection circuit of FIG. 5 and anaverage value of the TKIC register for each half-track, in the case ofthe outward direction search (BWFW=“L”).

FIG. 10 is a characteristic diagram showing the relation between thevelocity error of the velocity error detection circuit of FIG. 5 and anaverage value of the TKIC register for each half-track, in the case ofthe inward direction search (BWFW=“H”).

FIG. 11 is a circuit diagram showing a configuration of a conventionalvelocity error detection circuit of the track search control circuit inthe DVD system.

FIG. 12 is a characteristic diagram showing the relation between thevelocity error and the velocity error detection signal i.e. the value inFVCTR in the circuit of FIG. 11.

FIG. 13 is a characteristic diagram showing the relation between thevelocity error detection signal i.e. the value in FVCTR and the valuestored in the TKIC register in the circuit of FIG. 11, in the case ofthe outward direction search (BWFW=“L”).

FIG. 14 is a characteristic diagram showing the relation between thevelocity error detection signal i.e. the value in FVCTR and the valuestored in the TKIC register in the circuit of FIG. 11, in the case ofthe inward direction search (BWFW=“H”).

FIG. 15 is a waveform diagram showing the output timing for thedetection result of the velocity error by the velocity error detectioncircuit of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, referring to the drawings, an embodiment of the presentinvention will be described in detail.

FIG. 1 shows the block diagram of the DVD system according to anembodiment of the present invention.

An optical disc 1 serves as a storage medium, and is rotated by a discmotor 14.

An optical pickup 2 acts not only as signal writing means for recordingthe information data on the optical disc 1 in the form of digital dataand but also as signal extracting means for reading the information datarecorded in the optical disc 1. The optical pickup 2 emits a laser beamon the track of the optical disc 1. The track on the optical disc 1 isirradiated with the laser beam so that information data is recordedthereon. The optical pickup 2 detects a change of the radiant flux ofthe returning laser beam reflected by the track on the optical disc 1 toread the information data and outputs the information data as anelectric signal.

The RF amplifier 3 extracts from an output signal of the optical pickup2 a focusing error signal FE, the tracking error signal TE thatindicates the positional displacement between the track and the laserbeam, a readout signal RF, and the readout ripple signal RFRP.

A focus control loop comprises the RF amplifier 3, a focus servoequalizer 4, a focus actuator driver 6, and a focus actuator of theoptical pickup 2.

The focus servo equalizer 4 is for receiving the focusing error signalFE and performing gain compensation and phase compensation to secureopen loop gain and phase margin both necessary for the focus servo, andthe focus actuator of the optical pickup 2 is driven in accordance withthe output signal of the focus servo equalized 4 through the focusactuator driver 6.

By means of a feedback loop of the focus control so formed, the laserbeam emitted from the pickup 2 is controlled so as to focus on theoptical disc 1.

A tracking control loop comprises the RF amplifier 3, a tracking servoequalizer 5, a tracking actuator driver 7, and a tracking actuator ofthe optical pickup 2.

The tracking servo equalizer 5 is a circuit for receiving the trackingerror signal TE and performing gain compensation and phase compensationto secure open loop gain and phase margin both necessary for thetracking servo, and the tracking actuator of the optical pickup 2 isdriven in accordance with the output signal of the tracking servoequalizer 5 through the tracking actuator driver 7.

By means of a feedback loop of the tracking control so formed, the laserbeam emitted from the pickup 2 is controlled so as to be positioned onthe track on the optical disc 1.

In order to perform recording/reproduction on the whole area of theoptical disc 1, from the innermost to the outermost in the radialdirection, it is necessary to move the optical pickup 2 in the radialdirection of the optical disc 1. A motor and a sliding actuator are usedas a mechanism for moving the optical pickup 2, and a sending motorcontrol circuit 17 and a sliding actuator driver 18 are used to controlthe mechanism. The sending motor control circuit 17 drives the slidingactuator through the sliding actuator driver 18 and the sending motor 19based on the output signal of the tracking servo equalizer 5 and controlinput signal from the system controller 11.

A data sampling circuit-CD/DVD data signal processing circuit 8 is forreceiving the readout signal RF, binarizing the signal RF, extracting abit clock therefrom, extracting a synchronizing signal therefrom, andsubsequently decoding the binarized signal RF, and making a correctionusing correction RAM 9.

The header signal detection circuit 10, when reproducing the DVD-RAM,receives the readout signal RF, detects a header, and sends thedetection result to the system controller 11, which uses the detectionresult for servo control at a header portion and information extractionin the header. Moreover, at a changing point of a land/groove, byinverting the polarity of the tracking servo loop by controlling apolarity inverter circuit 20 in response to a land/groove changingsignal from the system controller 11, a change from the land to thegroove or vice versa is accomplished.

A disc motor control circuit 12 receives the synchronizing signalextracted by the data sampling circuit-CD/DVD data signal processingcircuit 8, and carries out CLV (constant linear velocity) control of thedisc motor 14 through a disc motor driver 13. Moreover, at the time ofCAV (constant angular velocity; i.e. constant rotation number)reproduction, the disc motor control circuit 12 receives a signal FGhaving a frequency proportional to the rotation number of the opticaldisc 1 that is generated by the disc motor driver 13 and the disc motor14, and carries out CAV control of the disc motor 14 based on the signalFG.

A track search control circuit 16 receives a track search directionsignal BWFW and a necessary number of tracks to be searched from thesystem controller 11 when performing the track search, and generatestrack traversing information of the laser beam spot from the trackingerror signal TE and the readout ripple signal RFRP. Then, the tracksearch control circuit 16 calculates the acceleration/decelerationenergy necessary for the track search from the track traversinginformation, the track search direction, and the number of tracks to besearched, and subsequently applies a signal indicative of theacceleration/deceleration energy to an input of the tracking servoequalizer 5 through an acceleration/deceleration energy adder 5 a actingas a track search velocity correction circuit. At this time, thetracking control loop used normally for the reproduction is made open byopening a switch 21.

The following circuits are formed on the same LSI chip: the focus servoequalizer 4; the tracking servo equalizer 5; the polarity invertercircuit 20; the acceleration/deceleration energy adder 5 a; the datasampling circuit•CD/DVD data signal processing circuit 8; the disc motorcontrol circuit 12; the track search control circuit 16; the sendingmotor control circuit 17; and the switch 21.

An operation concerning read/write of the optical disc in the DVD systemwill now be described briefly.

(1) The operation when information is read from the optical disc 1 is asfollows.

A signal read from the optical disc 1 by the optical pickup 2 isinputted into the RF amplifier 3 and the RF amplifier 3 extractstherefrom a focusing error signal FE, the tracking error signal TE, areadout signal RF, and the readout ripple signal RFRP.

The focus error signal FE and tracking error signal TE are inputted intothe focus servo equalizer 4 and the tracking servo equalizer 5,respectively, and compensated for the gain and phase, and inputted intothe respective actuator drivers 6 and 7, which drive the focus actuatorand the tracking actuator, respectively.

The readout signal RF is binary coded and undergoes the extraction ofthe bit clock and the extraction of the synchronizing signal, andsubsequently is demodulated and undergoes a correction with the use ofthe correction RAM 9, by the data sampling circuit•CD/DVD data signalprocessing circuit 8.

At the time of the DVD-RAM reproduction, the readout signal RF is alsosent to the header signal detection circuit 10 simultaneously, where theheader is detected. The detection result is sent to the systemcontroller 11 and is used for the servo control in the header portionand the information extraction in the header.

Moreover, at the changing point of the land/groove, a land/groove changesignal from the system controller 11 is sent to a polarity invertercircuit 20 in the tracking servo loop. By inverting the polarity of atracking servo loop with the polarity inverter circuit 20, the changefrom the land to the groove or vice versa is accomplished.

The synchronizing signal extracted in data sampling circuit•CD/DVD datasignal processing circuit 8 is sent to the disc motor control circuit12, and is used to carry out CLV control of the disc motor 14 throughthe disc motor driver 13. Moreover, when the CAV reproduction isperformed, the disc motor 14 and the disc motor driver 13 generate thesignal FG having a frequency in proportion to the rotation numbers ofthe optical disc 1, which is sent to the disc motor control circuit 12,and the disc motor 14 is controlled based on the signal FG.

In the case of the DVD movie, data that is corrected by the dataextracting circuit-CD/DVD data signal processing circuit is sent to thedata buffer circuit-MPEG video decoder/audio decoder processing circuit15, and is converted into a video output signal or an audio outputsignal.

On the other hand, in the case of the DVD-ROM, data that is corrected bythe data sampling circuit•CD/DVD data signal processing circuit 8 issent to the data buffer circuit 15, and subsequently is sent to a hostpersonal computer, etc.

The system controller 11 controls the control timing of each controlcircuit and the operation of the whole system. Moreover, headerinformation previously written in the optical disc 1 is read by theheader signal detection circuit 10 and the data sampling circuit-CD/DVDdata signal processing circuit 8.

(2) The operation when information is written in the optical disc 1 isas follows.

When information to be written is image information, a video signal issent to the data buffer circuit-MPEG video encoder and audio encoderprocessing circuit 15, and subsequently is added with ID data, a paritybit, etc. and undergoes ECC encoding and modulation in the CD/DVD datasignal processing circuit 8.

When information to be written is data, writing data is sent to thebuffer circuit 15, and subsequently is added with ID data, the paritybit, etc. and undergoes the ECC encoding and the modulation in theCD/DVD data signal processing circuit 8.

Moreover, a bit clock is generated from the disc information, and whilesynchronizing with the bit clock, the writing data after the modulationis sent to the RF amplifier 3, whose output is used to form pits on thedisc 1 with the pickup 2.

Here, referring to FIG. 2, the track search control circuit 16 of FIG. 1will be described in detail.

In the track search control circuit 16, the track pulse generationcircuit 22 generates a track pulse necessary for the track search basedon the tracking error signal TE, the readout ripple signal RFRP, and thetrack search direction BWFW.

The track counter 23 performs counting in such a way that the number oftracks to be jumped that is necessary for the track search and given bythe system controller 11 is subtracted with the number of track pulsesinputted by the track pulse generation circuit 22, and outputs a signalindicative of the number of the remaining tracks to the target track.

The velocity decoder 24 decodes the signal indicative of the number ofthe remaining tracks outputted by the track counter 23 and outputs atrack search target velocity signal corresponding to the number of theremaining tracks. In this occasion, as shown in FIG. 3, the track searchtarget velocity is controlled to a high velocity when the number of theremaining tracks is large, to a gradually reducing velocity as thepickup gets close to the target track, and lastly to such a velocity inwhich the tracking error signal falls into the tracking servo band widthwhen the pickup is getting to the target track.

The target velocity clock generation circuit 25 converts the targetvelocity data given by the velocity decoder 24 into the target velocityclock FVCK while referring to an oscillation frequency of a crystaloscillator (X′tal) 26 as a time standard. This target velocity clockFVCK is a signal having a frequency 16 times that of the target velocity(track traversing frequency). FIG. 4 shows the conversion characteristicof this target velocity clock generation circuit 25.

By comparing the target velocity clock FVCK given by the target velocityclock generation circuit 25 and the track pulse inputted by the trackpulse generation circuit 22, the velocity error detection circuit 27performs velocity error detection in a later-described way. The detectederror is modified by the track search direction signal BWFW and storedin the TKIC (tracking search velocity control) register 28 asacceleration/deceleration data in the tracking direction.

This acceleration/deceleration data stored in the TKIC register 28 ismultiplied by a summing-up coefficient K and inputted into theacceleration/deceleration energy adder 5 a for addition, which applies asignal indicative of the acceleration/deceleration energy in thetracking direction to the tracking servo equalizer 5.

FIG. 5 is the logical circuit chart showing one example of the velocityerror detection circuit 27 of FIG. 2.

FIG. 6 is the characteristic diagram showing the relation between thenumber of FVCK counts and the velocity error detection signals ACRRY andBCRRY in the velocity error detection circuit of FIG. 5.

FIG. 7 is a characteristic diagram of a decoder of the velocity errordetection circuit of FIG. 5.

FIG. 8 is the waveform diagram showing the velocity error detectionresult by the velocity error detection circuit 27 of FIG. 5 and theoutput timing for the velocity control output. This is an example in acase where the track search is performed in the outward direction.

FIG. 9 is a characteristic diagram showing the relationship between thevelocity error of the velocity error detection circuit 27 of FIG. 5 andan average value of the values stored in the TKIC register in eachhalf-track, in the case of the outward direction search (BWFW=“L”).

FIG. 10 is a characteristic diagram showing the relationship between thevelocity error of the velocity error detection circuit 27 of FIG. 5 andan average value of the values stored in the TKIC register in eachhalf-track, in the case of the inward direction search (BWFW=“H”).

Hereafter, referring to FIGS. 5 to 10, this embodiment will bedescribed.

FVACTR 51 is a first velocity error detection counter and measures themoving velocity of the light beam after the generation of the normaldirection on-track signal, and FVBCTR 52 is a second counter andmeasures the moving velocity of the light beam after the generation ofthe normal direction off-track signal.

A signal FVACLR is the on-track pulse signal and clears the counterFVACTR 51 at the time of being on-track, and a signal FVBCLR is theoff-track pulse signal and clears the counter FVBCTR 52 at the time ofbeing off-track.

A signal ACRRY becomes “H” level when the relative velocity between thetrack and the light beam after the generation of the normal directionon-track signal is lower than the target velocity, and a signal BCRRYbecomes “H” level when the relative velocity between the track and thelight beam after the generation of the normal direction off-track signalis lower than the target velocity.

A signal ACCP is an acceleration pulse and becomes “H” level when therelative velocity between the track and the light beam after thegeneration of the normal direction on-track signal and the relativevelocity between the track and the light beam after the generation ofthe normal direction off-track signal are both lower than the targetvelocity, and a signal BRKP is a deceleration pulse and becomes “H”level when the relative velocity between the track and the light beamafter the generation of the normal direction on-track signal and therelative velocity between the track and the light beam after thegeneration of the normal direction off-track signal are both higher thanthe target velocity.

In the velocity error detection circuit 27 shown in FIG. 5, the firstvelocity error detection counter (FVACTR) 51 and the second velocityerror detection counter (FVBCTR) 52 are time measurement circuits eachcomprising a 5-bit counter, respectively, and each is counted up by apulse of the target velocity clock FVCK given by the target velocityclock generation circuit 25 shown in FIG. 2.

The FVACTR 51 is cleared by an inverted signal of the on-track pulsesignal FVACLR (a pulse generated at the time of the track search in theon-track period) through an inverter circuit 60, and the FVACTR 51 iscleared by an inverted signal of the off-track pulse signal FVBCLR (apulse generated at the time of the track search in the off-track period)through an inverter circuit 61. The on-track pulse signal FVACLR and theoff-track pulse signal FVBCLR are generated by using the tracking errorsignal TE, the readout ripple signal RFRP and the track search directionBWFW so as to synchronize with the zero-crossing of the tracking errorsignal TE in the track pulse generation circuit 22 acting as the tracktraversing signal generation circuit shown in FIG. 2.

Therefore, the FVACTR 51 is able to count thirty two clocks (FVCKs), andwhen having counted sixteen clocks, the most significant bit outputACRRY becomes “H” level. In order to hold the counter output throughouta period after this output ACRRY becomes “H” level and until the FVACTR51 is cleared by the inverted signal of the on-track pulse signalFVACLR, a gate 54 of the target velocity clock FVCK is closed by aninverted signal of the output ACRRY through an inverter circuit 53.

Similarly to the above, the FVBCTR 52 is able to count thirty two clocks(FVCKs) and when having counted sixteen clocks, the most significant bitoutput BCRRY becomes “H” level. In order to hold the counter outputthroughout a period after this output BCRRY becomes “H” level and untilthe FVBCTR 52 is cleared by the off-track pulse signal FVBCLR, a gate 56of the target velocity clock FVCK is closed by an inverted signal of theoutput BCRRY through an inverter circuit 55.

It should be noted that the target velocity clock FVCK is a clock havinga frequency sixteen times that of the actual target velocity, and thatthe FVACTR 51 measures the half-track period after the generation of thenormal direction on-track signal with the target velocity clock FVCK andthe FVBCTR 52 measures the half-track period after the generation of thenormal direction off-track signal with the target velocity clock FVCK.

The time period in which the output ACRRY of the FVACTR 51 is “H” levelindicates a state in which it is detected that the moving velocity inthe half-track period after the generation of the normal directionon-track signal is lower than the target velocity, and thus the outputACRRY can be used as a flag indicating whether the velocity in thehalf-track period after the generation of the normal direction on-tracksignal is higher or lower than the target velocity.

Also, the time period in which the output BCRRY of the FVBCTR 52 is “H”level indicates a state in which it is detected that the moving velocityin the half-track period after the generation of the normal directionoff-track signal is lower than the target velocity, and thus the outputBCRRY can be use as a flag indicating whether the moving velocity in thehalf-track period after the generation of the normal direction off-tracksignal is higher or lower than the target velocity.

Furthermore, the two flags ACRRY and BCRRY are inputted into the ANDgate 57, which generates an acceleration flag ACCP when the movingvelocities in the half-track period after the generation of the normaldirection on-track signal and in the half-track period after thegeneration of the normal direction off-track signal are both lower thanthe target velocity. Moreover, the two flags ACRRY and BCRRY areinputted into a NOR gate 58, which generates a deceleration flag BRKPduring the time period in which the moving velocities in the half-trackperiod after the generation of the normal direction on-track signal andin the half-track period after the generation of the normal directionoff-track signal are both lower than the target velocity. That is, theacceleration flag ACCP and the deceleration flag BRKP each acting as anerror signal are generated in the AND gate 57 and the NOR gate 58,respectively, serving as error signal generation circuits.

Furthermore, the acceleration flag ACCP and deceleration flag BRKP aredecoded by the decoder 59 and stored in the TKIC register 28 shown inFIG. 2. As shown in FIG. 7, the decoder 59 decodes the acceleration flagACCP and deceleration flag BRKP in such a way that the maximumacceleration value or the maximum deceleration value is stored in theTKIC register 28 based on the acceleration or the deceleration.

In this embodiment, at a time when the necessity ofacceleration/deceleration is found out, the acceleration/decelerationenergy is immediately supplied. However, since at this moment of time,it is not yet known how much velocity error the moving velocity has, thedecoder 59 outputs any one of the three values, i.e., the maximumacceleration value, the minimum deceleration value, and zero, as shownin FIG. 7.

Therefore, when the velocity error detection circuit 27 applies a signalindicative of an acceleration energy or deceleration energy, thevelocity error detection circuit 27 applies the signal of accelerationenergy of a constant level or the signal of deceleration energy of aconstant level. However, as shown in FIGS. 9 and 10, an average supplyvalue of the energy for each track is linear with respect to thevelocity error.

In this way, by a process where the track search circuit outputs theacceleration/deceleration data exactly at a time when the track searchvelocity is turned out to be lower or higher than the target velocity(i.e., in the acceleration time, within a half-track period whendetection of the velocity error is started, and in the deceleration timeat the time when the succeeding half-track comes within the targetvelocity period after detection of the velocity error is started) andimmediately apply a signal indicative of the acceleration/decelerationenergy into the tracking servo equalizer (represented by the numeral 5in FIG. 1), the phase delay can be minimized and the track search can beperformed stably.

As mentioned in the foregoing, according to the track search controlcircuit and the optical disc drive equipped with a track search deviceincluding the track search control circuit of the present invention, itis possible to promptly reflect the judgment result of the track searchvelocity on the acceleration/deceleration data even at the multi-speedreproduction time where the relative velocity modulation caused by theeccentricity of the optical disc between the track and the objectivelens in the optical pickup becomes large, to thereby stably perform thetrack search operation.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A track search control circuit comprising: an optical pickup foremitting and moving a light beam in the radial direction of an opticaldisc to write information signal into the optical disc or read theinformation signal therefrom; a track traversing signal generationcircuit configured to detect, when the light beam emitted from theoptical pickup moves in the radial direction of the optical disc, thelight beam having traversed a track of the optical disc, and generate anormal direction on-track signal in an on-track period when the lightbeam traverses a zone of the track in a track search direction definedby a system controller and a normal direction off-track signal in anoff-track period when the light beam traverses a zone between thetracks; a first time measurement circuit configured to start timemeasurement at a time when the on-track signal is generated by the tracktraversing signal generation circuit; a second time measurement circuitconfigured to start time measurement at a time when the off-track signalis generated by the track traversing signal generation circuit; avelocity error signal generation circuit configured to detect an errorbetween a relative moving velocity of the light beam of the optical discto the track and a target velocity based on a measurement outputted bythe first time measurement circuit and a measurement outputted by thesecond time measurement circuit to generate an error signal; and acorrection circuit configured to correct the moving velocity of thelight beam in the radial direction based on the error signal generatedby the velocity error signal generation circuit.
 2. A track searchcontrol circuit according to claim 1, wherein the on-track signal andthe off-track signal are generated approximately at the time ofzero-crossing of a tracking error signal indicating a relativepositional displacement in the radial direction of the optical discbetween the track and the light beam emitted from the optical pickup. 3.A track search control circuit according to claim 2, wherein the tracktraversing signal generation circuit detects the light beam havingtraversed the track of the optical disc based on the tracking errorsignal and a readout ripple signal.
 4. A track search control circuitaccording to claim 1, wherein the track traversing signal generationcircuit detects the light beam having traversed the track of the opticaldisc based on a tracking error signal indicating a relative positionaldisplacement in the radial direction of the optical disc between thetrack and the light beam and a readout ripple signal.
 5. An optical discdrive comprising: an optical pickup for emitting a light beam on a trackof an optical disc, on which information is recorded, and receiving thereflected light from the track or the transmitted light therethroughwhile the optical disc is rotating, thereby extracting the informationand converting the information to an electric signal; a signalprocessing circuit configured to generate a tracking error signal thatindicates a relative positional displacement in the radial direction ofthe optical disc between the track and the light beam emitted from theoptical pickup and a ripple signal that indicates amplitude information,from the electrical signal output of the optical pickup at a time whenthe light beam emitted from the optical pickup moves in the radialdirection of the optical disc; tracking servo mechanism configured tocontrol the light beam emitted from the optical pickup in response tothe tracking error signal so that the light beam in the radial directionof the disc is positioned on the track; a track traversing signalgeneration circuit configured to detect that the light beam emitted fromthe optical pickup has traversed the track based on the tracking errorsignal and the ripple signal, and generate a normal direction on-tracksignal in an on-track period when the light beam traverses a zone of thetrack and a normal direction off-track signal in an off-track periodwhen the light beam traverses a zone between the tracks; a first timemeasurement circuit configured to start time measurement at a time whenthe on-track signal is generated by the track traversing signalgeneration circuit; a second time measurement circuit configured tostart time measurement at a time when the off-track signal is generatedby the track traversing signal generation circuit; a velocity errorsignal generation circuit configured to detect an error between a movingvelocity of the optical beam in the radial direction of the optical discand a target velocity based on a measurement output of the first timemeasurement circuit and a measurement output of the second timemeasurement circuit to generate an error signal; and a tracking velocitycorrection circuit configured to correct the moving velocity of theoptical beam in the radial direction by applying the error signal outputof the velocity error signal generation circuit to the tracking servomechanism.
 6. An optical disc drive according to claim 5, wherein thetracking velocity correction circuit starts to apply a signal indicativeof an acceleration energy corresponding to the error signal to thetracking servo mechanism in a half-track period after when the velocityerror signal generation circuit starts the error detection and a signalindicative of a deceleration energy corresponding to the error signal tothe tracking servo mechanism when a succeeding half-track comes in thetarget velocity period after when the velocity error signal generationcircuit starts the error detection.
 7. An optical disc drive accordingto claim 6, wherein the track traversing signal generation circuit, thefirst time measurement circuit, the second time measurement circuit, thevelocity error signal generation circuit, and the tracking velocitycorrection circuit are formed on the same semiconductor chip in a formof an integrated circuit.
 8. An optical disc drive according to claim 5,wherein the track traversing signal generation circuit generates theon-track signal and the off-track signal approximately at the time ofzero-crossing of the tracking error signal.
 9. An optical disc driveaccording to claim 8, wherein the track traversing signal generationcircuit, the first time measurement circuit, the second time measurementcircuit, the velocity error signal generation circuit, and the trackingvelocity correction circuit are formed on the same semiconductor chip ina form of an integrated circuit.
 10. An optical disc drive according toclaim 5, wherein the first time measurement circuit comprise a firstcounter which is cleared by the on-track signal, counts clock signalshaving a constant frequency higher than that of the on-track signal, andgoes into a hold status after generating a first flag output indicatingthat the moving velocity of the light beam after the generation of theon-track signal is lower than the target velocity; the second timemeasurement circuit comprises a second counter which is cleared by theoff-track signal, counts the clock signals, and goes into a hold statusafter having counted a specified number of clocks and subsequentlygenerating a second flag output indicating that the moving velocity ofthe light beam after the generation of the off-track signal is lowerthan the target velocity; and the velocity error signal generationcircuit, based on the first flag output and the second flag output,generates an acceleration flag when the moving velocity of the lightbeam after the generation of the on-track signal and the moving velocityof the light beam after the generation of the off-track signal are bothlower than the target velocity, and generates a deceleration flag whenthe moving velocity of the light beam after the generation of theon-track signal and the moving velocity of light beam after thegeneration of the off-track signal are both higher than the targetvelocity.
 11. An optical signal drive according to claim 10, wherein thetracking velocity correction circuit applies the signal indicative ofthe acceleration energy or deceleration energy of substantially aconstant level to the tracking servo mechanism during both theacceleration flag and the deceleration flag are logically set up.
 12. Anoptical disc drive according to claim 11, wherein the track traversingsignal generation circuit, the first time measurement circuit, thesecond time measurement circuit, the velocity error signal generationcircuit, and the tracking velocity correction circuit are formed on thesame semiconductor chip in a form of an integrated circuit.
 13. Anoptical disc drive according to claim 10, wherein the track traversingsignal generation circuit, the first time measurement circuit, thesecond time measurement circuit, the velocity error signal generationcircuit, and the tracking velocity correction circuit are formed on thesame semiconductor chip in a form of an integrated circuit.
 14. Anoptical disc drive according to claim 5, wherein the track traversingsignal generation circuit, the first time measurement circuit, thesecond time measurement circuit, the velocity error signal generationcircuit, and the tracking velocity correction circuit are formed on thesame semiconductor chip in a form of an integrated circuit.
 15. A tracksearch control circuit comprising: an optical pickup for emitting andmoving a light beam in the radial direction of an optical disc to writeinformation signal into the optical disc or read the information signaltherefrom; a signal processing circuit configured to generate a trackingerror signal that indicates a relative positional displacement in theradial direction of the optical disc between the track and the lightbeam emitted from the optical pickup and a ripple signal; a tracktraversing signal generation circuit configured to detect, when thelight beam emitted from the optical pickup moves in the radial directionof the optical disc to write the information signal into the opticaldisc or read the information signal therefrom, the light beam havingtraversed a track of the optical disc based on the tracking error signalindicating a relative positional displacement in the radial direction ofthe optical disc between the track and the light beam and the ripplesignal, and generate a normal direction on-track signal in an on-trackperiod when the light beam traverses a zone of the track in a tracksearch direction defined by a system controller and a normal directionoff-track signal in an off-track period when the light beam traverses azone between the tracks; a first time measurement circuit configured tostart time measurement at a time when the on-track signal is generatedby the track traversing signal generation circuit; a second timemeasurement circuit configured to start time measurement at a time whenthe off-track signal is generated by the track traversing signalgeneration circuit; a velocity error signal generation circuitconfigured to detect an error between a relative moving velocity of thelight beam of the optical disc to the track and a target velocity basedon a measurement outputted by the first time measurement circuit and ameasurement outputted by the second time measurement circuit to generatean error signal; and a correction circuit configured to correct themoving velocity of the light beam in the radial direction based on theerror signal generated by the velocity error signal generation circuit.16. A track search control circuit according to claim 15, wherein theon-track signal and the off-track signal are generated approximately atthe time of zero-crossing of the tracking error signal.
 17. An opticaldisc drive, comprising: a signal processing circuit configured togenerate a tracking error signal that indicates a relative positionaldisplacement in the radial direction of an optical disc between a trackand an optical beam from an optical pickup which emits the optical beamon the track of the optical disc, on which information is recorded, andreceive the reflected optical from the track or the transmitted opticaltherethrough while the optical disc is rotating, thereby extracting theinformation and converting the information to an electric signal, and aripple signal that indicates amplitude information, from the electricsignal output of the optical pickup at a time when the optical beamemitted from the optical pickup moves in the radial direction of theoptical disc; a tracking servo mechanism configured to control theoptical beam emitted from the optical pickup in response to the trackingerror signal so that the optical beam in the radial direction of thedisc is positioned on the track; a track traversing signal generationcircuit configured to detect that the optical beam emitted from theoptical pickup has traversed the track based on the tracking errorsignal and the ripple signal, and generate an on-track signal in anon-track period when the optical beam traverses a zone of the track andan off-track signal in an off-track period when the optical beamtraverses a zone between the tracks; a first time measurement circuitcomprising a first counter which is cleared by the on-track signal,counts clock signals having a constant frequency higher than that of theon-track signal, and goes into a hold status when counting a specifiednumber of clock signals, after generating a first flag output indicatingthat the moving velocity of the optical beam after the generation of theon-track signal is lower than the target velocity; a second timemeasurement circuit comprising a second counter which is cleared by theoff-track signal, counts the clock signals, and goes into a hold statuswhen counting a specified number of clock signals, after generating asecond flag output indicating that the moving velocity of the opticalbeam after the generation of the off-track signal is lower than thetarget velocity; and a velocity error signal generation circuitconfigured to generate, based on the first flag output and the secondflag output, as an error signal, generate an acceleration flag when themoving velocity of the optical beam after the generation of the on-tracksignal and the moving velocity of the optical beam after the generationof the off-track signal are both lower than the target velocity, andgenerate a deceleration flag when the moving velocity of the opticalbeam after the generation of the on-track signal and the moving velocityof the optical beam after the generation of the off-track signal areboth higher than the target velocity; and a tracking velocity correctioncircuit configured to correct the moving velocity of the optical beam inthe radial direction by applying the error signal output of the velocityerror signal generation circuit to the tracking servo mechanism; whereinthe tracking velocity correction circuit starts to apply a signalindicative of an acceleration energy or a deceleration energycorresponding to the error signal to the tracking servo mechanism in ahalf-track period after when the velocity error signal generationcircuit starts the error detection.
 18. An optical disc drive accordingto claim 17, wherein the on-track signal and the off-track signal aresubstantially at a constant level.
 19. An optical disc drive accordingto claim 18, wherein the track traversing signal generation circuit, thefirst time measurement circuit, the second time measurement circuit, thevelocity error signal generation circuit, and the tracking velocitycorrection circuit are formed on the same semiconductor chip in a formof an integrated circuit.
 20. An optical disc drive according to claim17, wherein the track traversing signal generation circuit, the firsttime measurement circuit, the second time measurement circuit, thevelocity error signal generation circuit, and the tracking velocitycorrection circuit are formed on the same semiconductor chip in a formof an integrated circuit.
 21. An optical disc drive, comprising: asignal processing circuit configured to generate a tracking error signalthat indicates a relative positional displacement in the radialdirection of an optical disc between a track and an optical beam from anoptical pickup which emits the optical beam on the track of the opticaldisc, on which information is recorded, and receive the reflectedoptical from the track or the transmitted optical therethrough while theoptical disc is rotating, thereby extracting the information andconverting the information to an electric signal, and a ripple signalthat indicates amplitude information, from the electric signal output ofthe optical pickup at a time when the optical beam emitted from theoptical pickup moves in the radial direction of the optical disc; atracking servo mechanism configured to control the optical beam emittedfrom the optical pickup in response to the tracking error signal so thatthe optical beam in the radial direction of the disc is positioned onthe track; a track traversing signal generation circuit configured todetect that the optical beam emitted from the optical pickup hastraversed the track based on the tracking error signal and the ripplesignal, and generate an on-track signal in an on-track period when theoptical beam traverses a zone of the track and an off-track signal in anoff-track period when the optical beam traverses a zone between thetracks; a first time measurement circuit comprising a first counterwhich is cleared by the on-track signal, counts clock signals having aconstant frequency higher than that of the on-track signal, and goesinto a hold status when counting a specified number of clock signals,after generating a first flag output indicating that the moving velocityof the optical beam after the generation of the on-track signal is lowerthan the target velocity; a second time measurement circuit comprising asecond counter which is cleared by the off-track signal, counts theclock signals, and goes into a hold status when counting a specifiednumber of clock signals, after generating a second flag outputindicating that the moving velocity of the optical beam after thegeneration of the off-track signal is lower than the target velocity;and a velocity error signal generation circuit configured to generate,based on the first flag output and the second flag output, as an errorsignal, generate an acceleration flag when the moving velocity of theoptical beam after the generation of the on-track signal and the movingvelocity of the optical beam after the generation of the off-tracksignal are both lower than the target velocity, and generate adeceleration flag when the moving velocity of the optical beam after thegeneration of the on-track signal and the moving velocity of the opticalbeam after the generation of the off-track signal are both higher thanthe target velocity; and a tracking velocity correction circuitconfigured to correct the moving velocity of the optical beam in theradial direction by applying the error signal output of the velocityerror signal generation circuit to the tracking servo mechanism, whereinthe track traversing signal generation circuit generates the on-tracksignal and the off-track signal approximately at the time ofzero-crossing of the tracking error signal.
 22. An optical disc driveaccording to claim 21, wherein the track traversing signal generationcircuit, the first time measurement circuit, the second time measurementcircuit, the velocity error signal generation circuit, and the trackingvelocity correction circuit are formed on the same semiconductor chip ina form of an integrated circuit.
 23. An optical disc drive, comprising:a signal processing circuit configured to generate a tracking errorsignal that indicates a relative positional displacement in the radialdirection of an optical disc between a track and an optical beam from anoptical pickup which emits the optical beam on the track of the opticaldisc, on which information is recorded, and receive the reflectedoptical from the track or the transmitted optical therethrough while theoptical disc is rotating, thereby extracting the information andconverting the information to an electric signal, and a ripple signalthat indicates amplitude information, from the electric signal output ofthe optical pickup at a time when the optical beam emitted from theoptical pickup moves in the radial direction of the optical disc; atracking servo mechanism configured to control the optical beam emittedfrom the optical pickup in response to the tracking error signal so thatthe optical beam in the radial direction of the disc is positioned onthe track; a track traversing signal generation circuit configured todetect that the optical beam emitted from the optical pickup hastraversed the track based on the tracking error signal and the ripplesignal, and generate an on-track signal in an on-track period when theoptical beam traverses a zone of the track and an off-track signal in anoff-track period when the optical beam traverses a zone between thetracks; a first time measurement circuit comprising a first counterwhich is cleared by the on-track signal, counts clock signals having aconstant frequency higher than that of the on-track signal, and goesinto a hold status when counting a specified number of clock signals,after generating a first flag output indicating that the moving velocityof the optical beam after the generation of the on-track signal is lowerthan the target velocity; a second time measurement circuit comprising asecond counter which is cleared by the off-track signal, counts theclock signals, and goes into a hold status when counting a specifiednumber of clock signals, after generating a second flag outputindicating that the moving velocity of the optical beam after thegeneration of the off-track signal is lower than the target velocity;and a velocity error signal generation circuit configured to generate,based on the first flag output and the second flag output, as an errorsignal, generate an acceleration flag when the moving velocity of theoptical beam after the generation of the on-track signal and the movingvelocity of the optical beam after the generation of the off-tracksignal are both lower than the target velocity, and generate adeceleration flag when the moving velocity of the optical beam after thegeneration of the on-track signal and the moving velocity of the opticalbeam after the generation of the off-track signal are both higher thanthe target velocity; and a tracking velocity correction circuitconfigured to correct the moving velocity of the optical beam in theradial direction by applying the error signal output of the velocityerror signal generation circuit to the tracking servo mechanism; whereinthe track traversing signal generation circuit, the first timemeasurement circuit, the second time measurement circuit, the velocityerror signal generation circuit, and the tracking velocity correctioncircuit are formed on the same semiconductor chip in a form of anintegrated circuit.